Man, when they said Riverdale would be a subversive take on the Archie Comics, they weren't messing around. From the opening moments of the pilot, the murder of Jason Blossom (Trevor Stiles) has driven so much story and exposed so many ugly sides of this small town at the center of the CW drama's first season and tonight, we finally found out who put the fatal bullet in the ginger's nogggin. But it was a long and winding road to the reveal, as one suspect after another piled up: Jughead (Cole Sprouse) found out that his father FP (Skeet Ulrich) confessed to the crime, Veronica (Camila Mendes) accused her incarcerated father of arranging the homicide from behind bars, Betty (Lili Reinhart) clearly worried her dad (Lochlyn Munro) had a hand it....even Kevin Keller's (Casey Cott) sheriff dad looked shady. In fact, it seems like the only person who was never a possibility was Archie (KJ Apa) himself!

In the end, the killer's identity hit closer to home than Jason's bereaved and bitchy twin sister Cheryl Blossom (Madelaine Petsch) could have ever imagined: Her own father, Clifford Blossom (Barclay Hope), had pulled the trigger after FP had abducted Jason during his staged death on the river and kept him locked up in the basement of the White Worm bar. It's a situation stickier than any Sticky Maple and one that will certainly shake the town to its core, especially now that the Coppers know that they are actually Blossoms by blood. Here, series creator Roberto Aguirre-Sacasa walks us through the decision to tell this almost Greek tragedy and hints at more reveals to come.

When did you land on Mr. Blossom being the killer?
About two weeks after we started the writer's room for Season 1. After meeting and talking about the characters and the stuff for two weeks, we had to go in and pitch to the studio and the network. The day before, I got a phone call and I can't remember if it was from the studio or the network, but they were like "...and you know that there's no way you're coming in without telling us who killed Jason Blossom!" [Laughs] And so what we did is we went in—we hadn't written the season, and things obviously change as you write and as you work with actors and you see what relationships develop—but we went in with three candidates who might have killed Jason. One was FP Jones, and one was [the never seen] Hiram Lodge and one was Clifford Blossom. We also even had it like Clue a little bit, a version where one of them was responsible, a version where two of them were responsible, and a version where all three of them did it.

But Cliff won out!
[Laughs] Just as we were developing the season, the noose started to tighten around Clifford more and more, but we really wanted to keep people guessing up to the reveal. Do you know what I mean?

You call it Clue, I was thinking more of like Murder on the Orient Express, where almost everyone had a hand in Jason's death. After he got off that boat to fake his death, everyone he dealt with was complicit in his demise. So you have FP, Mustang, then Clifford...
Yes, yes, yes, that's right, that's right.

Even to a point, Cheryl has to carry some of this blame.
And by the way, that's something that we're going to play very aggressively in the finale.

So what's next?
I am a true-crime fanatic and I love whodunnits. But what I really love is whydunnits. What happened that made it absolutely essential that Jason had to die by his father's hand? Which also, in the end, was the most powerful, the darkest and most transgressive way to kill him. And that's a big mystery that is answered in the finale. I think there's huge fallout. FP is still in jail, we have to figure that out. The mayor (Robin Givens) is having her big jubilee celebration, and now the wealthiest family in Riverdale is yet again besieged by an even darker scandal. The kids, they all watched the video of Jason's murder on that laptop and we really talk about that being the moment where innocence truly dies in Riverdale. There is a lot to the fallout that happens alongs with, of course, all of the characters' relationships. Those were put on pause a little bit this week just because there's so much to unravel and so much to solve.

There was such great emotional stuff. Cole in the phone booth was heartbreaking. And the kids watching the video! Lili Reinhart killed with those quiet, tears and Camila's face...it's only after the fact that you realize that they are watching a dad kill his own son.
It's like the most taboo thing.

So it's not because of the Cooper-Blossom connection...was Clifford's motive connected to his wife dragging Cheryl out to see all of the maple syrup stored in the barn?
Yes, it is connected to that. It is connected to that.

The character is described as 'a ruthless, handsome, charismatic businessman.'

And when did you guys decide to actually make the Coopers and Blossoms related by blood?
That was actually also in the first pitch. In the very first pitch, we revealed that part of the reason that the parents were going to be against this union is because they knew that Jason and Polly were related. We skinned the cat a little bit, we had Hal Cooper (Lochlyn Munro) knowing but Alice (Madchen Amick) didn't. And then the Blossoms knowing but kind of being okay with it, in that very gothic Fall of the House of Usher-Flowers in the Attic sort of way. I think when we pitched it, I avoided using the word "incest" and, again I can't remember if it was the studio or the network, someone said "But what you're talking about really is incest" and I was like, "Well, kind of!"

I love that Betty's reaction was more disgusted at being a Blossom than the fact that her sister is pregnant with a blood relative's twins.
[Laughs] Exactly, exactly. It's so dark.

And twisted! Now, does Cheryl's mom know or not?
That's a good question, does she know it? And what really happened in that bar? Those are the kind of questions that we play out in the last episode of the season. The finale is fully next-level, and not what you think. It will be be what you might expect from a finale.